Cathode ray tube



Dec. 1, 1936. F. J. G. VAN DEN BOSCH 2,062,538

CATHODE RAY TUBE Filed Jan. 23, 1936 INVENTOR HTTORNEY.

Patented Dec. 1, 1 936 UNITED STATES CATHODE BAY TUBE Francois JosephGerard Van den Bosch, London,

Application January 23, 1936, Serial No. 60,371 In Great BritainFebruary 13, 1935 4Claims.

A known form of cathode ray tube comprises a glass envelope in the formof a cylinder flaring out into a frusto-conical bulb, the inner face ofthe end wall of the bulb being coated with a substance, such aswillemite or calcium tungstate, which becomes fluorescent underbombardment by the cathode ray. Within the envelope are arranged, in theorder mentioned, a hot emitting cathode surrounded by an open-endedmetal cylinder, one or more anodes each in the form of a centrallyapertured disc,- a pair of horizontal and a pair of vertical deflectingplates.

Focussing of the ray on the fluorescent screen is achieved by suitablyadjusting the relative positions and potentials of these electrodes, andhitherto great difliculty has been experienced in reducing the size ofthe fluorescent spot on the screen to that required for such purposesas'high definition television.

It is an object of the present invention to provide means whereby thecathode ray may be more accurately and finely focussed upon the screen,giving at the same time an increased light intensity.

With this end in view, and in accordance with the invention, the platesof each of the horizontal and vertical deflecting pairs of plates arecoated on their adjacent faces with a substance such as an alkali metaloxide or iodide for example, strontium-oxide, barium-oxide, lime, iodideof calcium, calcium fluoride or caesium, which are known to emitelectrons easily when i even slightly heated. Preferably the plates ofeach of said pairs are parallel over a minor portion of their axiallength nearer the cathode and diverge outwardly from the cathode over amajor portion farther from the cathode.

By axis of the tube" is meant a line coincident with the centre-line ofthe beam of rays passing through the tube.

- A preferred form of the invention will now be described with referenceto the accompanying drawing, which is a perspective view of theelectrodes of a cathode ray tube constructed in accordance with theinvention.

A cathode ray tube comprises a cathode l, a Wehnelt cylinder 2surrounding the cathode, a flrst anode 3 in the form of a centrallyapertured disc, a second and similar anode 4, a pair of. vertical"deflecting plates 5 and 6 and a pair of horizontal deflecting plates 1and 8. The two pairs of plates 5, 6 and I, 8 are termed vertical" andhorizontal deflecting plates respectively, since they operate to deflectthe ray in the vertical and horizontal planes respectively.

The vertical plate I is earthed and the vertical deflecting potentialsare applied between the plates 8 and I. The horizontal plate 5 ismaintained at 200 volts negative with respect to plate 1, and thehorizontal deflecting potentials are applied between the plates 5 and 6.The second anode 4, the first anode 3, the Wehnelt cylinder 2 and thecathode I are maintained respectively at the following negativepotentials with respect to the plate I: 600 volts, 1200 volts, 1435volts and 1400 volts.

Adjacent faces of the plates 1, 8, and 5, 8, are coated with asubstance, such as potassium or strontium oxide or caesium which emitselectrons freely when a current is passed through the tube and theseplates are slightly heated. Preferably a current of 5 to 20milli-amperes is passed to these plates which then become slightlyheated. Each pair may consist of parallel rectangular plates or, asshown, each plate such as 5 may be formed from a rectangular plate bentalong a line 9 which is perpendicular to the tube axis Ill. The distanceII, I2 is about A; of the axial length of the plate 5. The plates ofeach pair are bent outwardly and disposed symmetrically about the axisHi.

It is found that by coating the plates as described, a sharply focussedand very small fluorescent spot can be produced on the screen, thefocussing efi'ect being increased by inclining the plates as shown inthe drawing.

The same eifect is obtained with soft tubes containing a rare gas suchas argon, neon, helium or a gas of low molecular weight such ashydrogen.

The relative potentials of the various electrodes may be variedconsiderably from those quoted above.

The adjacent faces of the plates of each pair I, 8 and 5, 6 may becoated either all over or, as shown, only upon thedivergent portions.

I claim:-

1. A cathode ray tube comprising a cathode, an anode, a pair of verticaldeflecting plates and a pair of horizontal deflecting plates, the platesof each of said pairs being coated, on their adjacent sides, with anelectron emitting substance.

2. A cathode ray tube comprising a cathode, an anode, a pair of verticaldeflecting plates and a pair of horizontal deflecting plates, the platesof each of said pairs being coated, on their adjac'ent sides, with anelectron emitting substance selected from strontium oxide, barium oxide,lime, calcium iodide, calcium fluoride and caesium.

3. A cathode ray tube comprising a cathode, an anode, a pair or verticaldeflecting plates and a pair of horizontal deflecting plates, the platesof each pair being parallel over a minor portion of their axial lengthnearer the cathode, diverging outwardly from the cathode 'over theremaining major portion further from the cathode and being coated ontheir adjacent taces with an electron emitting substance.

4. A cathode ray tube comprising a cathode. an anode, a pair of verticaldeflecting plates and a pair oi horizontal deflecting plates, the plates01' each pair being parallel over a minor portion 4 01' their axiallength nearer the cathode diverging outwardly from the cathode over theremaining major portion further from the cathode and being coated on theoutwardly divergent pottions or theiradjacent faces with an electronemitting substance.

FRANQOiS JQ SEPH GERARD VAN pm 30868.

